When Horowitz asked Firefox 3.5.11, 3.6.3 and 3.6.8 if there was an update, the browsers told him no newer editions were available. Firefox's up-to-date versions are 3.5.12 and 3.6.9, which Mozilla released a week ago when it patched 15 vulnerabilities, 11 of them labeled "critical."

Computerworld staffers reproduced the issue when they tried to update a copy of Firefox 3.6.6 on Windows Vista and Firefox 3.6.8 on a Mac.

Normally, older versions of Firefox will automatically receive an update with 24-48 hours after it's released, or when the user manually selects the "Check for Updates" from the Help menu.

Monday, Mozilla said it has stopped offering Firefox 3.5.12 and 3.6.9 because of a bug that crashed some users' machines after they'd updated, then restarted the browser.

"We've limited updates to Firefox 3.6.9 and Firefox 3.5.12 at this time as we evaluate some early feedback which indicates that a subset of our user base may be finding the releases unstable," said Michael Shaver, Mozilla's head of engineering, in an e-mail reply to questions yesterday.

According to an entry in Bugzilla, Mozilla's bug and code change database, the company began receiving a large number of crash reports after releasing Firefox 3.5.12 and 3.6.9 last week. The reports have come from machines on all three platforms -- Windows, Mac and Linux -- that Firefox supports, with the majority of them occurring during browser startup.

Mozilla did not say when it would switch updates back on.

Users who want to upgrade older versions of Firefox can download 3.5.12 or 3.6.9 from Mozilla's Web site, then install them manually overtop existing copies.